1. Field of the Invention
The field relates to inflatable devices such as air bags and more particularly to an air bag having a burst point control envelope with particular application to stun grenades.
2. Description of the Related Art
U.S. Pat. No. 8,117,966 taught a non-pyrotechnic stun grenade for generating loud, explosive sound by inflation to rupture of an inflatable bag. To make the point of rupture consistent from bag to bag and to achieve target noise levels within a limited time period the '966 patent proposed to construct a single layer inflatable bag with a rupture seam. Upon inflation the rupture seam parted abruptly at a particular and predetermined degree of tension on the seam. The rupture seam parted at a design volume of the bag and pressure within the bag to produce an N-wave. The explosive sound produced consistently met a minimum target volume level. Although the '966 patent provided for a non-pyrotechnic, compressed air, inflation source the patent suggests that pyrotechnic gas generation more readily produced high gas flow rates than compressed gas sources.
The use of chemical reactions to generate gas generators for inflation of automotive air bags is known. One issue addressed during the development of such air bags was the type of gas generator to use. Among the concerns was the byproducts produced by the chemical reactions or combustion of the fuel source used to generate the gas.
A popular contemporary gas generator for automotive applications is a mixture of sodium azide (NaN3), potassium nitrate (KNO3) and silicon dioxide (SiO2). An exothermic (heat producing) decomposition of sodium azide into nitrogen gas and sodium can be initiated by exposure of the compound to 300° C. The free nitrogen gas inflates the bag while the potassium nitrate reacts with the sodium in a second reaction to produce potassium oxide (K2O), sodium oxide (Na2O) and more free nitrogen (N2). A final reaction translates the reactive potassium oxide and sodium oxide compounds into more stable byproducts by a reaction with the silicon dioxide to produce potassium silacate and sodium silicate (K2O3Si and Na2O3Si). These are chemically stable compounds which pose no known environmental and health threat. See Gas Laws Save Lives: The Chemistry Behind Airbags, Casiday, R. and Frey, R. (2000). In addition, the initiating materials are not hygroscopic as water absorption can slow or stop gas generating reactions limiting the shelf life of units. Alternative pyrotechnic formulations for a gas generator may make use of potassium nitrite (KNO2). Such fuel sources result in reactions which are highly exothermic and can produce higher temperatures than the reaction based on sodium azide.
Construction of an inflatable bag which ruptures at a consistent degree of inflation to produce predictable noise levels using an exothermic chemical reaction to produce the inflation gas poses issues not present when a compressed air source is used. In contrast, where a compressed gas source is used for inflation the temperature of compressed gas falls upon expansion.